The present invention relates to a high harmonic current reducing apparatus in a power converter with a converting apparatus comprising rectifiers and being connected with an AC power source, for reducing high harmonic currents generated in the AC circuit of the converter during commutating periods.
The power converter of this type has a high efficiency of power conversion and thus has been applied to many systems of various power conversion capacity. An example of such applications is an electric car.
In general, switching operation of turning-on or turning-off the current flow is an essential function of the controlled or non-controlled rectifiers used in the power converter.
For this reason, the AC current abruptly changes at the time of commutation. The abrupt change of AC current is accompanied by high harmonic currents. The high harmonic currents produce noise voltages in other electric systems such as, for example, communication cables. That is, it provides a problem of inductive interference.
In this kind of power converter, it is impossible to avoid the problem of inductive interference caused by commutation. One conventional countermeasure for this problem is that some modification for reducing noise voltage is made in the apparatus which are subject to such an inductive interference. Another countermeasure is that a plurality of power converters are used with the DC sides being connected in series to gradually increase the output voltage so that the noise voltage is reduced. The former countermeasure is disadvantageous in that the problem of inductive interference arising from the power converter is not solved, but adversely additional high cost for such a countermeasure is necessary. The latter countermeasure using a plurality of power converters may solve that problem to some extent, but it is insufficient to reduce high harmonic components of an AC current which also cause inductive interference.
The factors determining the high harmonic components of the AC current in the power converter have been studied by the inventors.
The equivalent interference current J.sub.p causing the inductive interference is related to the effective value of high harmonic components of the AC current in the power converter and is represented by the following equation ##EQU1## where Sn designates the weight of the respective frequencies and is called the noise evalulation coefficient, and In the effective value of high harmonic components.
The inventors noticed that there might be some relation between the equivalent interference current J.sub.p and the AC current of the power converter, because, in the operation (1), the effective value In of high harmonic components may be developed into a Fourier series, if the primary current waveform is known.
Further, the inventors notices that the equivalent interference current J.sub.p changed in accordance with the change of the control angle of the rectifier.
A test was conducted on the relationship between the change of the AC current waveform of the converter and the variation of the equivalent interference current J.sub.p, when the control angle of the rectifier was changed. As a result of the test, it was ascertained that there was a particular relationship therebetween. That is, it was found that the change of the AC current waveform due to the commutation in accordance with the control angle in the case of a small equivalent interference current J.sub.p was slower than that in case of a large equivalent interference current J.sub.p.
This relationship shows that, if the change of the AC current waveform at the commutation is slow, the equivalent interference current J.sub.p causing an inductive interference is effectively reduced.